The human spine is comprised of bony vertebrae separated by softer discs and has been the subject of surgical interest for several decades. Since the 1950s spine surgeons have used metallic hardware to increase the success of stabilization surgery. Many spinal disorders lead to spinal instability and surgeons have used the process of bone healing to fuse spinal segments using bone grafts from either the patient or other sources. The addition of spinal hardware in spine surgery has significantly increased the success rate from these bone healing operations commonly known as fusion operations. Spinal hardware or instrumentation has been a developing field of spine surgery and has involved merging the hardware with various portions of the human vertebra.
The human vertebras are commonly divided into four major regions with a corresponding numbering system. The cervical spine has seven vertebra, the thoracic spine has twelve, and the lumbar spine has five. The sacrum, or tailbone as it is commonly referred to, is considered one mechanical bone segment but is actually made up of many fused segments during human gestation. The human vertebral diagram (FIGS. 1-2) shows the basic vertebral features, the vertebra 40 includes a vertebral body (41), the pedicle (42), the transverse process (43), the facet joint (44), the lamina (45), and the spinous process (46). All of these features can be found on vertebra in the cervical, thoracic and lumbar spine.
In the 1970's advances were made in spine instrumentation by the development of screw stabilization systems that utilized the pedicle of the vertebra for anchoring of the screws. Surgeons would use these systems by passing screws from the posterior approach into the vertebral body. These systems were commonly used as paired pedicle screw systems and the increased stabilization strength led to fewer complications from fusion failure. More recently stabilization systems utilizing the anterior approach to the spine have been developed and have used various screw stabilization technologies. The anterior stabilization systems utilize screw stabilization frequently to hold anterior spinal hardware in place. The anterior screw stabilization systems previously described get most of their holding power from the cortical bone in the anterior and lateral portion of the vertebral body. One of the most devastating complications from spinal stabilization implantation is the phenomena of hardware “pullout”. This term refers to when the mechanism relied upon to hold either anterior or posterior stabilization hardware fails to hold the hardware in place and the hardware becomes dislodged. With anterior hardware the failure results in the hardware lifting off of the spine as the screw fixation pulls through the cortical bone of the vertebral body. In posterior hardware cases, the pedicle screws pull out of the pedicle and the hardware attached posteriorly becomes dislodged pulling away from the spine posteriorly. In either instance, the stabilization mechanism whether it be screws in the anterior area or hooks or screws in the posterior area fail to provide the stability for which they were intended and reoperation becomes more likely usually involving extension of the stabilization to involve even more levels of the spine. Often the areas of attachment where the pullout occurred are no longer usable for a stabilization point because of the damaged bone texture in that area. Technology that reduces this pullout phenomenon is needed to reduce the hardware failure rate and subsequent fusion failure and reoperation rate.
As intraoperative imaging technology and endoscopic techniques have advanced, so too has the ability of the surgeon to form a more precise relationship when using spine instrumentation and the tools with which it is inserted. Over the years these have advanced from customary anatomical landmarks combined with the eye of the surgeon through the development of intraoperative radiographs, fluoroscopic techniques, and more recently intraoperative volumetric computer-assisted navigational technology and endoscopic technology. As these imaging technologies have advanced, accordingly, spinal instrumentation must advance in terms of the accuracy of hardware placement previously considered only possible with large incisions and increased surgical complication risk.
In addition to hardware pullout problems in healthy patients, one of the most challenging medical conditions facing spine surgeons is the patient with spinal instability as well as osteoporosis. Osteoporosis is a medical condition which lowers bone density and makes screw stabilization less successful by reducing the strength with which the screw holds into the bone substance. Screw pullout and hardware failure are significantly more common in patients with osteoporosis. Up to this point spinal stabilization systems for patients with osteoporosis have relied on supplementing the screw stabilization with special manufacturing processes on the surface of the screw to allow faster integration of the screw into the vertebra during bone healing. Other supplementary procedures have involved the introduction of cement into screw holes in osteoporotic patients to increase the strength of the screw relationship with the bone.